Potential energy functions Recommendations

Only fools and well-trained scientists keep on wondering why 

Some of the questions quoted in the beginning of chapter 3, or just implicit in the text, can now be answered in what follows, I do my best to guide colleagues who want to get started. 

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The interatomic potentials define the force field parameters that contribute to the lattice energy of a relaxed or energy minimized structure. The fundamental question is how reliable is a force field The force field used in evaluating a potential function must be consistent and widely applicable to all similar systems. It must be able to predict the crystal properties as measured experimentally. Two main approaches, namely empirical and semi-empirical, are usually employed in the derivation of potential parameters. Empirical derivations involve a least square fitting routine where parameters are chosen such that the results achieve the best correlation with the observed properties. The semi-empirical approach uses an approximate formulation of the quantum mechanical calculations. An example of such an approximation is the electron gas method [57] which treats the electron density at any point as a uniform electron gas. The following is the analytical description of the potential energy function and interatomic potentials we recommend for use in simulation of zeolites and related system. 

In view of the problems in describing accurately the interactions between water molecules, theoretical predictions of thermophysical properties have not been quantitatively very successful for water and steam. An investigation by Thoen-Hellemans Mason (1973) of the mumal consistency of the thermal conductivity and viscosity data at low density for steam, which does not require detailed knowledge of the inter-molecular pair potential energy function, showed that the thermal conductivity data in the skeleton tables at that time were consistent with the viscosity data within the given tolerances, but it was considered that the thermal conductivities were a few percent too low. However, more recent recommendations for the thermal conductivity data of steam (Sengers et al. 1984) are in fact lower than those earlier values. 

Use a shifted function only to reproduce reported results. Since a shifted dielectric potential affects the entire potential energy surface, it is not recommended. 

The work described in this review has concentrated upon the equilibrium geometries of each complex. It is frequently the case that improperly balanced theoretical approaches can offer a reasonable description of this particular structure. However, more complete understanding of the nature of the H-bond or fitting of an empirical function for later use in simulations requires the calculation of extended regions of the potential energy surface. It is here that the above recommendations become especially important. 

Intermolecular potential functions have been fitted to various experimental data, such as second virial coefficients, viscosities, and sublimation energy. The use of data from dense systems involves the additional assumption of the additivity of pair interactions. The viscosity seems to be more sensitive to the shape of the potential than the second virial coefficient hence data from that source are particularly valuable. These questions are discussed in full by Hirschfelder, Curtiss, and Bird17 whose recommended potentials based primarily on viscosity data are given in the tables of this section. 

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